It is known to limit asymmetrical loads in taking power for a plurality of consumer loads out of a multi-phase AC power grid in that the single-phase consumer loads are distributed over the individual phases of the multi-phase AC power grid. This measure, however, only provides levelling of the partial powers flowing via the individual phase conductors of the multi-phase AC power grid with regard to the statistical average.
In grid connection points displaying high impedances towards a connected AC power grid, voltage drops with regard to the grid voltage occur upon taking high powers from the AC power grid. It is known to compensate this effect by a variable transformer. In case of an asymmetrical load on the individual phases, however, the voltage differences resulting between the phases at the grid connection point displaying high impedances towards the AC power grid can not be compensated by means of a variable transformer.
As already indicated, undesired asymmetrical power distributions also occur In single-phase feeding electric energy into a multi-phase AC power grid. From DE 10 2006 003 904 A1 it is known to counter-act these asymmetries in that electric powers fed by a plurality of single-phase inverters into an AC power grid are evenly distributed over the phases of the AC power grid and that upon breakdown of one inverter feeding into one phase, the powers of the other inverters feeding in the other phases are limited.
An inverter system for individually feeding into the phases of a three-phase AC power grid is known from WO 2006/084294 A1. Here, a plurality of single-phase inverters are, at their input ends, connected to a power bus into which a plurality of photovoltaic power generators feed. The power fed by the photovoltaic power generators into the power bus changes with the solar radiation. The individual inverters are combined or connected in parallel for feeding the power presently available on the power bus into the AC power grid with a suitable low number of inverters to limit power losses. To the end of avoiding unbalanced loads on the phases, the single-phase inverters feeding power are pooled in three groups, i.e. in one group per phase.
The assignee of the present disclosure sells a bi-directional battery inverter called “Sunny Island”, which may be used as a grid former for island networks and which compensates for asymmetrical loads on the phases of the island networks by taking power from the un-loaded phases and outputting power to the loaded phases. This power compensation, however, has power losses within the bi-directional inverter as a consequence.
U.S. Pat. No. 4,177,508 discloses an apparatus for use in balancing an asymmetrical load which is supplied from a three-phase network. The apparatus is provided with an inverter for generating a three-phase output current system on its AC side which is fed with phases reversed to the network. The DC side of the inverter, in turn, is fed from a DC source with a DC current corresponding to the maximum asymmetry power to be balanced. Preferably, the DC source is a rectifier connected to the network. Thus, the entire known apparatus is only provided for balancing the asymmetrical load, and it means considerable costs and power losses.
The case of an extreme unbalanced load due to taking power at the grid connection point via one phase while simultaneously feeding power into the AC power grid via another phase, which has been mentioned at the beginning, is a disadvantage independently of the unbalanced load provoked in the AC power grid, if self-consumption of the electric power provided by an inverter has advantages. These advantages generally include that a self-consumption of locally generated electric power does not load the AC power grid. Additional financial advantages may result, if the electric energy taken from the AC power grid is more expensive than the electric power fed into the AC power grid. A financial advantage may, however, also occur, if an incentive for self-consumption of locally generated electric energy is higher than the difference between the payment for the electric energy fed into the grid and the price of electric energy taken from the grid. This is the case according to the German Erneuerbare-Energien-Gesetz in the version valid since Jan. 1, 2009 (EEG 2009). If the self-consumption has financial advantages, the case of feeding electric power via one phase while simultaneously taking electric power via another phase of the AC power grid is to be avoided by all means.
EP 2 114 002 A1 discloses a three-phase power conversion device connected between output terminals of a solar battery and a three-phase power system. The conversion device comprises a three-phase inverter circuit and single-phase inverters connected in series with AC output lines of the three-phase inverter circuit. The three-phase inverter circuit outputs a reverse-polarity voltage pulse during a period corresponding to a half cycle of a system voltage. A power burden born by the individual single-phase inverters in each half cycle is made approximately zero, and the individual single-phase inverters make a correction for subtracting a common voltage from target output voltages of individual phases during the period when the reverse-polarity voltage pulse is generated. There is no reference to balancing any loads on individual phases of the three-phase power system.
There still is a need for a method, an apparatus and an inverter for levelling partial powers at a grid connection point between a multi-phase AC power grid comprising a plurality of phase conductors, on the one hand, and a unit including a multi-phase inverter, which feeds electric energy into the AC power grid, plus consumer loads connected to the AC power grid, on the other hand, the partial powers flowing via the individual phase conductors, by which a self-consumption of locally generated power is optimized and by which asymmetrical loads on a multi-phase AC power grid at the grid connection point are reduced in general.